Cobalt-based superalloys are used for various components in gas turbine engines where the components are exposed to high temperatures and pressures for extended periods of time. A typical application is the high-pressure turbine vanes, where the temperature of the component can rise to about 2000.degree. F. Under these conditions, the component is expected to retain its shape and strength long enough to provide economical operation of the engine without unduly frequent service or replacement requirements.
Typically, cobalt-base superalloy components have been produced by precision casting, which provides a near-net shape component requiring only minimal machining operations to reach final configuration. To provide additional protection for the material from the high temperatures and corrosive environment due to combustion processes, a protective coating is commonly applied. Components with adequate properties for the desired applications have been successfully produced with no heat treatment other than the heating cycle incurred as part of the coating process, which is effectively a precipitation heat treatment of the as-cast material.
After extended service, some of these components incur damage, due, e.g., to erosion, thermal mechanical fatigue-induced cracking caused by the frequent cycling between ambient and operating temperatures, or creep, which causes the airfoils to bow or the platforms to twist away from their original positions, with a resultant change in the operational characteristics of the vanes. A method of repair of this damage which has proved successful entails the use of a mixture of powders in which one of the powders melts at a temperature lower that the melting point of the damaged component and then isothermally solidifies by the diffusion of a melting point depressant into the material of the damaged component. See, for example, U.S. Pat. No. 4,008,844 issued to Paulonis, et al., and U.S. Pat. No. 4,726,101 issued to Draghi, et al., both of common assignee with the present invention, which are incorporated herein by reference. The temperature to which the component is exposed during the repair cycle is typically about 2050.degree.-2150.degree. F.
It is accordingly an object of the present invention to provide a repair method for cobalt-base superalloy gas turbine engine components which provides improved mechanical properties compared to the prior art repair process, particularly the 1% creep life at engine operating temperatures.
It is another object to provide a method of adding material to the surface of a component which is worn or distorted to restore the component to its original dimensions.
It is a further object of the invention to provide a method for the heat treatment of cobalt-base superalloys which improves the mechanical properties.
Finally it is an object of the invention to provide a method for the rejuvenation of cobalt-base superalloy gas turbine engine components whose mechanical properties have been degraded by exposure to engine operating temperatures for extended periods of time.